1. Field of the Invention
This invention relates to coaxial transverse electromagnetic wave resonators.
2. Prior Art
A transverse electromagnetic wave resonator (hereinafter "TEM resonator") is an electromagnetic filter which is used to discriminate against all but one electromagnetic frequency. Coaxial resonators are described in U.S. Pat. No. 4,207,548 to Graham et al., and U.S. Pat. No. 2,637,782 to H. Magnuski. The resonator is basically a cylindrical can containing a central conductor. The outer can has an input electrode at which an electrical signal is introduced, having a range of frequencies. The can also has an output electrode at which a single frequency appears, depending on the length of the central conductor. The central conductor is often adjustable in length to enable frequency tuning. Refer to the Graham et al. reference for the remainder of this section, except as otherwise directed to the Magnuski reference, especially for the identity of reference numerals. The outer conductor 1 is a cylindrical can, having input and output terminals 4 and 5 respectively. The conductor 1 contains a fixed tubular outer conductor 20 which includes therein a slidable inner plunger 9. A rod 11 is fixed to plunger 9 and can be rotated to advance plunger 9 downward through conductor 20 or conversely, can be rotated to shift plunger 9 upward through conductor 20. To the extent that plunger 9 advances beyond the end of conductor 20, the apparent length of the central conductor is increased, tuning the frequency of resonance of the filter. Movement of plunger 9 is impelled by a rod 11 which is made of a metal having low electrical conductivity such as Invar.
The outer conductor has a cavity therein which can be considered to be electrically equivalent to a length of coaxial cable that is shorted from its inner conductor to the outer conductor (or shield) at one end and left open on the other end. At the shorted end, the voltage on the inner conductor equals the shield voltage, which is defined as zero, or ground potential. If a current develops on the inner conductor, it will have a maximum value at the short. At the open end, the current on the inner conductor is zero, and the voltage between the inner and outer conductor is at a maximum. The distance between these events on a cable is directly related to a distance a voltage maximum travels in a second (the wave velocity) and the frequency of the wave. The ratio of the velocity to frequency is defined as the wavelength, and it is also the physical distance between two wave maxima in a continuously repeating wave.
In the structure of the filter, the short must occur at the shorted end and the open must occur at the open end. The frequency and the velocity of the wave, mutually independent conditions, determine the distance between the open and the short for a given wavelength. At a given length between the open and the short, a discrete primary wavelength will resonate, having a current maximum at the short and a current minimum at the open. Since the velocity of the wave is set by the material between the inner and outer conductor, resonance will occur only at discrete frequencies determined by the ratio of the velocity of the wave in the cable to the resonance wavelengths. Thus the structure functions as a frequency selective device or resonator. The most basic resonator is defined as a quarter wave resonator. A quarter wave resonator has exactly one current maximum and one current minimum, separated by a distance equal to one-quarter wavelength. The details of such a resonator are described in the Magnuski reference. The length of the central conductor of a quarter wave resonator should be adjusted to be exactly one-quarter of the wavelength of the desired resonance frequency.
There are an infinite number of resonances in addition to the basic quarter wave field pattern that can occur in a coaxial cavity TEM resonator which is grounded at one end and open at the other. The current along the inner conductor of the resonator is difficult from the basic quarter wave resonance in that there is an additional current maximum for each resonance above the basic resonance. In a quarter wave resonator, the only maximum occurs at the shorted end at the base of the fixed section, (20 in Graham et al.) In a three quarter wave cavity, there are two maximum field points, one at the short and one at half wavelength distance from the short. Thus there are two maximum current points along the inner conductor of a three quarter wave cavity, yet the conditions of the unit having a current maximum at the shorted end, and a current minimum on the open end are still met. This is called a harmonic mode or a higher-order mode of operation. The length of the central conductor of a three-quarter wave cavity should be adjusted to be three fourths of the wavelength of the desired resonance frequency. When the coaxial resonator is used in an environment in which the temperature varies over a wide range, the inner conductor length must be held constant by some type of temperature compensation device. Magnuski teaches such compensation.
Refer to the Magnuski patent for the remainder of this paragraph. When the temperature of the entire resonator increases equally, rod 46 expand, causing the length of the overall inner conductor (stub 44 and plunger 45) to initially increase. At the same time, a compensating tower 51 expands in the opposite direction. Tower 51 is mechanically connected to a threaded assembly 49 which holds threaded rod 46 in place. The expansion of the tower counters the expansion of the rod, thus keeping the length of the inner conductor virtually unchanged. The drift in the frequency of resonance due to a temperature change is calculated as: EQU F.sub.t /F.sub.o =K*(L.sub.tower *A.sub.tower -L.sub.rod *A.sub.rod)*T
Where
F.sub.t is the frequency drift of the cavity; PA0 F.sub.o is the resonance frequency; PA0 K is the change in frequency normalized to Fo versus the change in inner conductor length; PA0 L.sub.tower is the length of the tower; PA0 L.sub.rod is the length of the rod; PA0 A.sub.tower is the linear coefficient of expansion of the tower material; PA0 A.sub.rod is the linear coefficient of expansion of the rod material; and PA0 T is the change in temperature.
As an example, consider a resonator having a copper tower 51 and a rod 46 made of a low expansion alloy known as INVAR.
Accordingly, A.sub.tower is 9.3 ppm/degree F.; A.sub.rod is 0.86 ppm/degree F. EQU F.sub.t /F.sub.o =K*(9.3*L.sub.tower -0.86*L.sub.rod)*T
Chose a tower height such that L.sub.tower =0.092*L.sub.rod, and it can be seen that F.sub.t =0. Therefore, the resonator will maintain a constant inner conductor length during ambient temperature changes and the drift in resonance frequency will be zero. If the tower and the rod are at the same temperature, there will be no frequency drift. However, in applications in which very high radio wave power levels are filtered, the assumption of equal tower and rod heating is invalid. If the input signal is of high power, the resistance heating in the central conductor can be significant. At the points along the conductor where maximum magnetic fields exist, there also maximum current also occur are localized heating is at a maximum. In the case of a quarter wave resonator, the peak currents occur on the fixed section 20 of the central conductor, also known as the stub. Since changes in the length of the fixed stub does not alter the overall length of the inner conductor, a heatup of the stub does not greatly alter the resonance frequency. The fixed stub is also in good thermal contact with tower and shield 1, further reducing the effects of thermal changes. Plunger 9, in contrast, is not generally in contact with any heat sink. Rod 11 is generally made of INVAR, a very poor heat conductor. Rod 11 is long and of small cross-section, reducing its ability to transfer heat from plunger 9.
In FIG. 1, inner plunger 9 is separated from stub 20 by a plurality of spacers labeled 19 and reference point B. These spacers 19 are generally of a plastic material and serve to prevent electrical contact between stub 20 and plunger 9. Spacers 19 conduct heat poorly.
Refer to FIG. 4 of Magnuski. In this Figure, the filter has a stub 44, and a movable plunger 55, both connected together electrically via fingers 56. The Magnuski device does not have the spacers 19 of Graham et al. Fingers 56 are metal and therefore conduct heat.
The Magnuski device is said to be "fingered" while the Graham et al. device is said to be "unfingered". Generally speaking, the fingered device enjoys better heat conduction between the stub and the plunger.
Both in the case of fingered and unfingered resonators, the mechanism by which heat is transferred away from the movable inner plunger 9 is mostly heat conduction in the gas surrounding the inner conductor. Very little heat is transferred by conduction through the stub (via fingers if present) or through the controlling rod 11.
In the case of a quarter wave resonator, most of the heat deposited in the inner conductor is deposited along the length of the inner conductor comprised by the fixed stub because the current maximum occurs at the short. In the case of the three quarter wave resonator, there is a current maximum which occurs along the portion of the inner conductor which is comprised by the movable plunger. A large amount of heat is deposited in the plunger, which has poor heat communication with the fixed stub, or with any other heat path leading to the tower. Consequently, a three quarter wave resonator may demonstrate poor response to temperature changes in that the resonance frequency will drift during temperature changes, both in fingered and unfingered configurations.
It is an object of this invention to improve the thermal stability of resonance cavities, especially three quarter wave resonance cavities.